The invention relates to a method of inspecting the welding of a sealed closure plug of a nuclear reactor fuel element.
Nuclear reactors, such as those cooled by pressurised water, include a core made up of fuel assemblies in which energy in the form of heat is produced during the operation of the reactor.
Each of the fuel assemblies is generally constituted of a bundle of parallel fuel elements retained in the framework of the fuel assembly. Each of the fuel elements includes a tubular sleeve made from a material that is poor at absorbing neutrons, such as a zirconium alloy, in which are stacked nuclear fuel pellets, for example sintered pellets of uranium oxide (UO2). The tubular sleeve is closed at both ends by a plug which has a cylindrical part which is inserted coaxially into an end part of the sleeve. The plug and the sleeve are then welded together along a circular line in a plane substantially perpendicular to the axis of the sleeve and the plug.
The fabrication of fuel elements requires many successive operations of filling the sleeve with the fuel pellets, fitting and welding the plugs and introducing a pressurised inert gas such as helium into the sleeve sealed by the plugs. Many inspection operations must be carried out during all steps of fuel element fabrication so that fuel elements are obtained with zero defects.
In particular, the welds between the plugs and the ends of the sleeve must be rigorously inspected.
The plugs are welded in a welding station including a sealed chamber containing an inert gas atmosphere into which part of the end of the sleeve with a plug inserted in it is introduced. The sleeve and the plug are coupled to a system for rotating them about their common axis inside the sealed welding chamber. The welding is effected by melting parts of the sleeve and the plug disposed one against the other in a joint plane perpendicular to the axis of the fuel element. A welding system such as a TIG welding torch or a laser beam welding head is used to melt the plug and the sleeve in the welding area. The electrode of the TIG welding torch or the beam from the laser welding head is disposed in the plane of the joint and substantially perpendicular to the axis of the fuel element. The plug has a shoulder between the cylindrical part inserted into the sleeve and a larger diameter part which remains outside the sleeve. Because of the accuracy with which the end surfaces of the sleeve and the plug are machined, the contact between the end part of the sleeve and the shoulder on the plug cannot be perfect and in practice the joint plane consists of the edge surface at the end of the sleeve and the surface of the shoulder on the plug, which are separated by a small annular interstice. When the weld is effected by melting the sleeve and the plug in the joint plane, the interstice must be closed and entirely filled with molten metal to obtain a perfect seal of the joint between the plug and the sleeve of the fuel element.
To obtain a proper weld it is necessary for the joint plane as just defined to be perfectly located relative to the welding axis, which is the axis of the electrode in the case of TIG welding or the axis of the beam in the case of laser welding.
After the weld is executed, it is necessary to verify that the plug and the sleeve were melted all around the circular joint line, inside the joint plane, and that there is no discontinuity in the weld, which could lead to a defective seal.
What is more, to avoid reducing the productivity of the production line, the welds between the plugs and the sleeves of the fuel elements must also be inspected in such a way that the fuel element fabrication time is not unduly increased. It is also desirable to allow for producing a fast diagnosis indicating whether the fuel element can be accepted.
Optical inspection of fuel element welds based on digitized images obtained by a scanning camera coupled to an image digitizer system has been proposed, for example in U.S. Pat. No. 5,602,885.
From each digitized image, a matrix is obtained of values of the reflective power of each pixel of the image, arranged in columns and rows of the image. A mean value of the reflective power of the pixels of the image is calculated and compared to the reflective power of each pixel. If the values for a particular number of adjoining pixels, corresponding to an area of the weld having the minimum size of a defect that can be detected, depart from the mean value by an excessive amount, the presence of an unacceptable welding defect is diagnosed.
A method of the above kind is used in an inspection station separate from the welding station, which means that it is necessary to pass the welded fuel elements from the welding station to the inspection station. The handling time between the welding station and the inspection station and the time needed to perform the inspection are therefore added to the fuel element fabrication time.
It is also necessary to provide rotational control means to inspect the weld by taking images along the circular weld line.
What is more, the above method provides no way of monitoring the positions of the plug and the sleeve before welding or of effectively testing the conformance of the weld, regardless of the type of welding employed.
The object of the invention is therefore to propose a method of inspecting the welding of a sealed closure plug of a fuel element for a nuclear reactor, the fuel element including a tubular sleeve enclosing a plurality of nuclear fuel pellets stacked in the axial direction of the sleeve and two sealed closure plugs having a cylindrical part inserted coaxially into an axial end part of the sleeve, a plug being welded in a welding station by melting the sleeve and the plug along a circular line in a joint plane perpendicular to the axis of the sleeve and the plug by a welding arrangement directed radially relative to the circular line in the joint plane of the sleeve and the plug, which are coupled to a rotation arrangement for rotating them about their common axis, inspection being effected by processing digitized optical images of areas of the fuel element adjoining the circular joint line and distributed along the periphery of the fuel element, this method checking that the plug is fitted correctly to the sleeve of the fuel element before welding, and checking the conformance of the weld, at the welding station, and in masked time, during the operation of welding the fuel element.
To this end before welding, the plug and the sleeve being in the welding position at the welding station, the sleeve and the plug are rotated about their common axis with the aid of the rotation arrangement, images are taken along the periphery of the fuel element to obtain digitized images, the digitized images are analysed to determine the position of the joint plane, and the rotation of the fuel element is verified, it is deduced whether it is possible to perform the welding, and if the welding is performed, after welding the plug to the sleeve of the fuel element, images are taken of the fuel element in position at the welding station, along the periphery of the fuel element, in the vicinity of the joint line, to obtain digitized images, and the digitized images are analysed to check the conformance of a weld made along the joint line.